Staphylococcus aureus is an opportunistic bacterial pathogen causing substantial morbidity and mortality. Antibiotic resistance is common in S. aureus, of which methicillin resistance is the resistance of greatest clinical consequence. Infections due to methicillin-resistant S. aureus (MRSA) cause longer hospital stays, more deaths, and are more expensive to treat than infections due to methicillin-susceptible S. aureus (MSSA) and uninfected controls. MRSA isolates can be molecularly characterized using pulsed-field gel electrophoresis (PFGE). PFGE has found that one strain of MRSA, the USA300 MRSA strain, is the most common cause of community-associated MRSA infections. Reasons for the success of this clone are not known, but given that USA300 MRSA isolates have a number of unique virulence factors, there could be an increase in morbidity and mortality associated with this shift. Few studies have evaluated USA300 MRSA as the cause of bacteremic infections. The overall objective of this evaluation is to identify risk factors and outcomes associated with bacteremic infections caused by USA300 MRSA. Specific Aims: 1) To determine whether the mortality in adults with bacteremic infections due to USA300 MRSA is greater than the mortality of a) adults with bacteremic infections due to non-USA300 MRSA, and b) adults without bacteremic infections due to S. aureus. 2) To determine the risk factors for bacteremic infections due to USA300 MRSA among a) adults with bacteremic infections due to S. aureus, and b) all adults. The risk factors of interest are injection drug use and homelessness. Methods: These studies will use cohort and case-control study designs and PFGE to identify USA300 MRSA. Significance: These aims promote the mission of the CDC to promote health and quality of life by preventing and controlling infectious diseases. Aim #1 will investigate the relationship among pathogen genetic factors and infection outcome in human subjects. Aim #2 focuses on high-risk populations for developing these infections. Both areas have been identified as priority research in Advancing the Nation's Health: A Guide to Public Health Research Needs: 2006-2015 established by the CDC, on p. 29 and 30, respectively. [unreadable]